The exhaust products of internal combustion engines are known health hazards to human beings, animals as well as plant life. The pollutants are, in general, non-burnt hydrocarbons, carbon monoxide, nitrogen oxides, as well as residual amounts of sulfur and sulfurous compounds. Exhaust catalysts have to meet stringent requirements with respect to light-off performance, effectiveness, long-term activity, mechanical stability as well as cost effectiveness in order to be suitable for vehicle application. The pollutants of non-burnt hydrocarbons, carbon monoxides as well as nitrogen oxides have been successfully treated by contact with multifunctional, noble metal catalysts which are capable of converting a high percentage of the pollutants into less harmful products of carbon dioxide, water (steam) and nitrogen. However, the sulfur and sulfurous compounds present in fuels and, in turn, in exhaust product, have been known to poison the noble metals resulting in lessening their catalytic effectiveness and life.
The “catalytic converter” used to convert the harmful pollutants into non-harmful gases, usually consists of three components, that is, the catalytically active metal, the support on to which the active metal is dispersed, and a substrate on to which the support is applied or “washcoated”.
The catalytic metals that are useful to cause effective conversion of harmful pollutants, like carbon monoxide, nitrogen oxides, and non-burnt hydrocarbons under the varying conditions encountered, are noble metals, usually the metals of the platinum group, such as platinum, palladium, rhodium and mixtures thereof. These noble metal catalysts are well known in the art and are more fully described in, for example, DE-05 38 30 318.
The noble metal is typically supported on high surface area inorganic oxides, such as high surface area alumina particles. The high surface area alumina is applied or “washcoated” onto a ceramic or metallic substrate, such as in the form of a honeycomb monolith or wire mesh or the like structure. It is also possible to apply the noble metals onto the support after washcoating the support material onto the monolith.
Nanocrystalline alumina is used as a catalyst support due to its high specific surface area and good thermal resistance to coarsening and sintering at elevated temperatures. However, alumina undergoes a strong interaction with sulfur and sulfurous compounds present in fuels and, in turn, in exhaust product, which results in the storage of SO4− at the surface of alumina. When so adsorbed, the sulfurous compounds are known to poison noble metal catalysts, especially those formed with platinum metal, causing reduction in activity and effective life of the catalyst system.
Silica has little interaction with sulfur and sulfurous compounds and does not show the ability to store sulfate. However, silica does not exhibit the hydrothermal stability required to form effective emission control catalyst supports and, therefore, is not a desirable catalyst support material for such applications. As such, it has been found to be desirable to modify the alumina surface with silica in order to combine the structural characteristics of alumina and chemical characteristics of silica.
WO 2008/045175 discloses a structure comprising a porous alumina particulate having silica cladding on its surface made by forming an alumina particulate into an aqueous slurry, mixing a silica precursor material with the slurry, treating the mixture with acid to form an aqueous suspension of treated alumina particles, washing the suspension to remove alkali metal materials, spray drying the suspension to provide dry particles, and then and calcining the dry particles to form a high surface area alumina having silica cladding on its surface.
It is desired to form an alumina catalyst support that is capable of enhancing the activity of noble metals in the conversion of carbon monoxide and hydrocarbon materials to carbon dioxide and water while exhibiting high tolerance to the presence of sulfur and sulfurous compounds by a simpler process.
It is further desired to form an alumina catalyst support capable of enhancing the activity of noble metals, especially platinum metal, to convert noxious emission products of internal combustion engines, especially diesel engines, to more environmentally benign products and to exhibit such activity over an extended life because of its enhance tolerance to the presence of sulfur and sulfurous compounds and to provide improved properties compared to prior alumina catalyst support materials.